With increasing radio-frequency (RF) spectral occupancy it is becoming increasingly important for electronic support measures (ESM) systems to monitor the radio-frequency spectrum from well below 1 GHz to nearly 100 GHz in real time. Photonic architectures for signal down-conversion (subsampling) and disambiguation are enabling technologies for real-time wideband signal monitoring which could provide lower size-weight-power-cost (SWaP-C) and reduced latency in next-generation military ESM and civilian applications.
From sampling theory it is well known that, when the bandwidth of an incoming signal exceeds one-half of the digitizer sampling rate, aliasing of the signal will occur. In all-electronic systems, specifically folding analog-to-digital converters (ADCs), the frequency ambiguity resulting from this aliasing process may be resolved in several ways. Most simply, an anti-aliasing filter may be placed before the ADC thereby limiting the input bandwidth to a known range. This directly removes any frequency ambiguity. Alternatively, two ADCs operating at two distinct sampling rates may be utilized. By using a lookup table, the unique mapping of the aliased signals from the two ADCs may be determined to reconstruct the original input signal (see U.S. Pat. No. 6,031,869, incorporated herein by reference). A final technique is to sample the input waveform using two ADCs where an intentional known delay is introduced prior to one of the ADCs. By measuring the additional phase change introduced by this additional delay the linear relation between phase and frequency may be inverted to determine the original input frequency (see U.S. Pat. No. 5,109,188, incorporated herein by reference). In optically subsampled (downconverting) ADC architectures, the first technique of applying a known bandwidth anti-aliasing filter is the only method which is equivalent to testing the optical ADC with a known input signal in the absence of a filter.
There are several limitations of these techniques. First, all are limited in the RF bandwidth to which they may be applied—the analog bandwidth of virtually all electronic ADCs is limited to approximately twice the sample rate. Thus, to monitor a frequency bandwidth approaching 100 GHz would require either multiple parallel downconvert-and-digitize channels or frequency-swept architectures. The use of appropriate anti-aliasing filters would necessarily need to be applied in either of these architectures. The latter two techniques require the use of two ADCs per-channel which increases the complexity and cost substantially when addressing such a broad bandwdith. While the use of a known delay is conceptually quite simple, from a system perspective delay variations in the RF chain or thermal fluctuations can limit the accuracy of the technique.